1. Field of the Invention
This application relates generally to the deposition of silicon-containing materials in semiconductor processing. More particularly, this application relates to the deposition of substitutionally-doped silicon-containing films by chemical vapor deposition using trisilane and a dopant source.
2. Description of the Related Art
The electrical properties of semiconductors such as silicon (Si), germanium (Ge) and alloys thereof (SiGe) are influenced by the degree to which the materials are strained. For example, silicon exhibits enhanced electron mobility under tensile strain, and silicon-germanium (SiGe) exhibits enhanced hole mobility under compressive strain. Methods of enhancing the performance of semiconductors are of considerable interest and have potential applications in a variety of semiconductor processing applications. As is well known, semiconductor processing is most commonly employed for the fabrication of integrated circuits, which entails particularly stringent quality demands, but such processing is also employed in a variety of other fields. For example, semiconductor processing techniques are often employed in the fabrication of flat panel displays using a wide variety of technologies and in the fabrication of microelectromechanical systems (MEMS).
A number of approaches for inducing strain in Si— and Ge-containing materials have focused on exploiting the differences in the lattice constants between various crystalline materials, e.g., Ge (5.65 Å), Si (5.431 Å) and carbon (3.567 Å for diamond). In one approach, thin layers of a particular crystalline material are deposited onto a different crystalline material in such a way that the deposited layer adopts the lattice constant of the underlying single crystal material. For example, strained SiGe layers may be formed by heteroepitaxial deposition onto single crystal Si substrates. Because the Ge atoms are slightly larger than the Si atoms, the deposited heteroepitaxial SiGe follows the smaller lattice constant of the Si beneath it and thus is compressively strained to a degree that varies as a function of the Ge content. Typically, the band gap decreases monotonically from 1.12 eV for pure Si to 0.67 eV for pure Ge as the Ge content in the SiGe increases. In another approach, tensile strain is introduced into a thin single crystalline silicon layer by heteroepitaxially depositing the silicon layer onto a strain-relaxed SiGe layer. In this example, the heteroepitaxially deposited silicon is strained because its lattice constant follows the larger lattice constant of the relaxed SiGe beneath it. The tensile strained heteroepitaxially deposited silicon typically exhibits increased electron mobility. In these approaches, the strain is developed at the substrate level before the device (e.g., a transistor) is fabricated.
Strain may be introduced into single crystalline Si-containing materials by substitutional doping, e.g., where the dopants replace Si in the lattice structure. For example, substitution of germanium atoms for some of the silicon atoms in the lattice structure of single crystalline silicon produces a compressive strain in the resulting substitutionally doped single crystalline silicon material because the germanium atoms are larger than the silicon atoms that they replace. A tensile strain may be introduced into single crystalline silicon by substitutional doping with carbon, because carbon atoms are smaller than the silicon atoms that they replace. See, e.g., Judy L. Hoyt, “Substitutional Carbon Incorporation and Electronic Characterization of Si1-yCy/Si and Si1-x-yGexCy/Si Heterojunctions,” Chapter 3 in “Silicon-Germanium Carbon Alloy,” Taylor and Francis, N.Y., pp. 59-89, 2002, the disclosure of which is incorporated herein by reference.
In situ doping is often preferred over ex situ doping followed by annealing to incorporate the dopant into the lattice structure because the annealing may undesirably consume thermal budget. However, in practice in situ substitutional carbon doping is complicated by the tendency for the dopant to incorporate non-substitutionally during deposition, e.g., interstitially in domains or clusters within the silicon, rather than by substituting for silicon atoms in the lattice structure. See, e.g., the aforementioned article by Hoyt. Non-substitutional doping also complicates substitutional doping using other material systems, e.g., carbon doping of SiGe, doping of Si and SiGe with electrically active dopants, etc. As illustrated in FIG. 3.10 at page 73 of the aforementioned article by Hoyt, prior deposition methods have been used to make crystalline silicon having an in situ doped substitutional carbon content of up to 2.3 atomic %, which corresponds to a lattice spacing of over 5.4 Å and a tensile stress of less than 1.0 GPa. However, prior deposition methods are not known to have been successful for depositing single crystal silicon having an in situ doped substitutional carbon content of greater than 2.3 atomic %.
Thus, there is a need for improved methods to accomplish in situ substitutional doping of Si-containing materials. Desirably, such improved methods would be capable of achieving commercially significant levels of substitutional doping without unduly sacrificing deposition speed, selectivity, and/or the quality (e.g., crystal quality) of the deposited materials.